Research And Implementation Of Cascaded DSTATCOM

The inductive-resistive load and impulse load used in daily life and industrial production put distribution system to the test of power quality issues, such us reactive power pollution and voltage flicker. As an effective means of power quality regulation, DSTATCOM has become research focus and represents the developing direction of DFACTS. Related theories and technologies of cascaded STATCOM for distribution system were researched in the thesis.Currently, three-phase bridge inverter, which has disadvantages of high output voltage THD, low voltage and power level, difficult to implementation individual control, and so on, is mostly used as the main circuit of DSTATCOM. In view of the disadvantages of three phase inverter, an idea of applying cascaded multilevel inverter to DSTATCOM was carried out in the thesis.The basic operation principle of DSTATCOM was analyzed and two laws were released: the reactive power exchange between DSTATCOM and power system could be controlled by adjusting DSTATCOM output voltage amplitude; the active power exchange between DSTATCOM and power system could be controlled by adjusting the phase difference between DSTATCOM output voltage and the system voltage. Expression of reactive power, active current and reactive current absorbed by DSTATCOM and the output voltage of DSTATCOM were obtained by analyzing the DSTATCOM mathematic model.To achieve better dynamic performances and compensation accuracy, two kinds of current controller were designed. The simulation result demonstrated that the proportional and repetitive controller based composite control method could not only ensure the dynamic performances but also improve the steady-state performances of DSTATCOM.DC capacitor voltages balance, which influences DSTATCOM's compensation performance and operation safety, is a special problem of cascaded multilevel inverter. Mechanism of cascaded DSTATCOM DC capacitor voltages unbalance was analyzed and a conclusion of DC capacitor voltages could be balanced by changing the active power absorbed by chain link inverters was obtained. Based on this conclusion, a distributed control strategy for DC capacitor voltages balancing control was carried out.To verify the parameters of current controller designed and the distributed control system based DC capacitor voltages balancing method, a 30kVar cascaded DSTATCOM prototype was developed. The design procedure, including hardware design and software design, was illustrated in detail. Furthermore, particular solutions to some engineering practical problems, such as arithmetic of current directly control and synchronization of phase-shifting carriers, were also discussed in the thesis. Experimental results demonstrated that the prototype not only had a function of compensating distribution system reactive power but also had good performances in reactive current command tracking and DC capacitor voltages balancing control.